1. Field of the Invention
The present invention relates to a clutch drum of a multi-plate clutch used in an automatic transmission and the like, and a multi-plate cultch having such a clutch drum. Further, the present invention relates to a method for manufacturing such a clutch drum.
2. Related Background Art
In general, a wet type multi-plate clutch is incorporated into an automatic transmission used in an automatic vehicle and the like. Such a clutch includes a clutch drum as a cylindrical rotary member having one open end. The clutch drum is connected to an input member. As shown diagrammatically in FIG. 6, the clutch drum 100 is provided at its interior with plural friction plates 105 and separator plates 106 which are alternately arranged along an axial direction. Further, a piston 107 is disposed within the clutch drum 100.
In the multi-plate clutch so designed, when operating oil pressure is applied to the piston, the friction plates and the separator plates are urged by the piston to be closely engaged with each other. As a result, the clutch drum connected to the input member, the friction plates, the separator plates and an output member are rotated integrally. By such an operation of the clutch, torque of an engine is transmitted from the input member to the output member.
The clutch drum comprises a disc portion and a cylindrical portion extending vertically from an outer periphery of the disc portion. An inner peripheral surface of the cylindrical portion is provided with splines into which the friction plates or the separator plates are fitted. That is to say, the friction plates or the separator plates can be shifted along the splines in an axial direction.
In many cases, such splines are formed by form rolling rather than cutting in order to maintain strength and to reduce the cost. By forming the clutch drum in this way, a clutch drum with splines having high accuracy and high strength can be manufactured.
As mentioned above, although the multi-plate clutch is set in the clutch drum, a radius R of a tip of each of splines of the clutch drum is decreased in order to increase a contact area between the splines of the clutch drum and the splines of the multi-plate clutch as much as possible to transmit the torque. For example, such an example is disclosed or illustrated in Japanese Patent Application Laid-open No. 2000-205294 and in FIG. 5 of this application.
In the general form rolling, as shown in FIG. 5, a cup-shaped clutch drum 100 is worked by the form rolling by using an apparatus comprising a mandrel to which a shaping tool 102 with a tooth profile is mounted, and a pressure roller 101, thereby forming splines in an inner periphery of the clutch drum 100.
A cup-shaped work is mounted on the mandrel to which the shaping tool is mounted and which is rotatingly driven. When a single pressure roller or plural pressure rollers are shifted in an axial direction, metal flow forming is caused between the mandrel or the shaping tool and the single pressure roller or the plural pressure rollers. In this case, a thickness of the work is decreased and a length of the work is increased and also a tooth profile is formed on an inner diameter surface of the work, thereby forming the clutch drum.
In this case, particularly, tooth roots of the tooth profile of the mandrel or the shaping tool and corresponding tooth tips of the tooth profile of the clutch drum are subjected to excessive stress, with the result that, for example, in an area A shown in FIG. 5, the tooth root of the tooth profile shaping tool on the mandrel is damaged or cracked and the tooth tip of the drum is scratched.